The amount of radiant energy emitted by a heated body is known to be proportional to the temperature of the body and may be calculated using the Stefan-Boltzmann Law and/or Planck's radiation formula. Conversely, the temperature of a heated body may be calculated by measuring the amount of radiation emitted by that body and relating the amount of radiation so measured to the temperature through the use of the radiation formula. Most conventional pyrometers have utilized this principle of operation. However, the accuracy of these devices has been limited because they have employed estimated emittance factors in their radiation formula calculations.
The use of estimated emittance factors has not generally been a problem since most applications for pyrometers do not require a high degree of accuracy. However, in some fabrication processes for semiconductor wafers, the diffusion characteristics of the dopants used are highly temperature dependent and therefore in order to control the diffusion of such dopants into the thin layers of materials which make up these wafers, a precise knowledge of the temperature of the semiconductor materials is required. In such processes, it is desirable to be able to measure the temperatures of the materials within + or -5.degree. C. in order to properly control diffusion effects. This level of accuracy is considerably beyond the capabilities of most conventional pyrometers.
Furthermore, the use of pyrometry techniques in the measurement of semiconductor wafer temperatures is complicated by the fact that the environment surrounding these wafers is frequently flooded with light from the radiative elements employed in heating and maintaining the temperature of the wafers during processing. Additionally, the measurement of wafer temperatures is rendered yet more difficult by the substantial variations in the emittance factors of such wafers due to interference effects arising from interactions between the emissions from the various layers of the wafers themselves. Many conventional pyrometers cannot be used in this environment because their readings would be seriously contaminated by stray radiation and would be significantly affected by errors in the estimated emittance factors they employ.
It is, therefore, an object of the present invention to provide a method and apparatus for pyrometry which provide more accurate temperature readings than previously used techniques.
It is another object of the present invention to provide a method and apparatus for pyrometry in which the emittance factor associated with the body whose temperature is being measured is accurately derived.
It is a further object of the present invention to provide a method and apparatus for pyrometry which is especially adapted for precisely measuring the temperatures of semiconductor materials during wafer fabrication processes and avoids the contamination problems associated with the radiation present in the fabrication environment.